範圍和陰影

變量綁定有一個範圍，並且被約束在一個塊中。塊是語句的集合括在大括號 {} 內，所以, 變量陰影 是被允許的。

fn main() {
    // This binding lives in the main function
    let long_lived_binding = 1;

    // This is a block, and has a smaller scope than the main function
    {
        // This binding only exists in this block
        let short_lived_binding = 2;

        println!("inner short: {}", short_lived_binding);

        // This binding *shadows* the outer one
        let long_lived_binding = 5_f32;

        println!("inner long: {}", long_lived_binding);
    }
    // End of the block

    // Error! `short_lived_binding` doesn't exist in this scope
    println!("outer short: {}", short_lived_binding);
    // FIXME ^ Comment out this line

    println!("outer long: {}", long_lived_binding);
}